(1) Field of the Invention
The present invention relates to flexible coupling means that accommodate angular and axial movements while connecting together two rotary mechanical parts. The coupling means may, for example, connect a transmission shaft to a mechanical part such as a rotorcraft turbine engine. The invention also provides a mechanical transmission including the flexible coupling means.
Consequently, the invention lies in the technical field of transmitting power from one mechanical part to another, in particular mechanical parts of a rotorcraft.
(2) Description of Related Art
Most presently-manufactured rotorcraft have at least one turbine engine with a free turbine. Power is then taken from a low pressure stage of the free turbine, which stage is mechanically independent of the compressor assembly and of the high pressure stage of the engine. The free turbine of such an engine generally rotates at a speed in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a speed-reducing gearbox is needed for its connection with the main rotor of the rotorcraft, since the speed of rotation of the rotor lies substantially in the range 200 rpm to 400 rpm: this is the main power transmission gearbox, often referred to as the “main gearbox” or by the initials MGB.
Under such conditions, the engine is connected to the MGB of the rotorcraft via at least one transmission shaft rotating about its axis of rotation at a speed that is often faster than 5000 rpm.
Likewise, a rotorcraft may include a tail rotor driven by at least one power transmission shaft in order to control the yaw movement of the rotorcraft.
In general, a transmission shaft needs to be securely fastened by coupling means to the mechanical members that it is to connect together.
Furthermore, those coupling means must enable the transmission shaft to transfer the power developed by one mechanical part to another under conditions that might be extreme, e.g. when the parts that are connected together are not properly in alignment with each other.
Means for coupling a transmission shaft to a mechanical part are then dimensioned to accommodate axial and angular misalignment between the transmission shaft and the mechanical part.
Coupling means are known that accommodate such misalignments, said means being provided with a first annular part and with a second annular part.
Each annular part comprises a flexible annular disk of small thickness extending radially from a base to an outer periphery. The base is secured to fastener means for fastening it to one of the rotary parts, such as a transmission shaft.
Each annular disk is sometimes referred to as a “diaphragm”. The term “diaphragm” is therefore used below for convenience.
Under such circumstances, the two annular parts are connected to each other solely via the outer peripheries of their diaphragms. Those outer peripheries may be fastened together by welding, or by using bolts and lock nuts, for example.
Diaphragm coupling means provide a solution that is advantageous in terms of connecting together rotary parts that might be misaligned.
Such diaphragm coupling means possess a relatively high level of reliability because they have few components. Nevertheless, diaphragm coupling means present diaphragms of small thickness that can lead to a certain amount of fragility if ever the nominal axial and angular misalignments that were taken into consideration during a design stage are exceeded.
Document U.S. Pat. No. 4,133,188 proposes flexible diaphragm coupling means seeking to mitigate the effects of said diaphragms breaking.
Those coupling means comprise a pair of diaphragms for transmitting torque between two rotary parts while accommodating axial and angular misalignment between those two parts.
Furthermore, the coupling means include an emergency torque transmission device making use of longitudinal splines.
While the diaphragms are connected together, torque is transmitted between the connected-together parts via the diaphragms.
In contrast, coupling is provided by the emergency device in the event of the diaphragms breaking.
Those coupling means are advantageous. Nevertheless, the coupling means comprise a relatively large number of mechanical components, thereby making manufacture complex.
Documents U.S. Pat. No. 5,407,386 and EP 0 026 681 also describe coupling means having an emergency device with splines.
Document U.S. Pat. No. 4,560,364 describes coupling means provided with a first member fastened to a first rotary part and a second member fastened to a second rotary part. The first member includes a first diaphragm fastened to a second diaphragm of the second member.
Furthermore, the first member is fastened to the first rotary part via a bolt. The bolt has a threaded shank co-operating with the first rotary part, a hexagonal head, and a shoulder arranged between said head and said threaded shank.
The second member also includes a plate for fastening to the second rotary part.
Under such circumstances, the coupling means are provided with an insert that co-operates with said shoulder. The insert is fastened by a disk that is interposed between the fastener plate of the second member and the second rotary part.
The shoulder has two projections representing male splines that co-operate with corresponding female splines in the insert.
Document U.S. Pat. No. 5,588,917 describes connection means having two torque transmission paths, one of the paths including splines.
Document US 2010/0093449 relates to diaphragm coupling means having means for limiting axial misalignment.
Document WO 2010/022862 is remote from the technical field of the invention. That document WO 2010/022862 describes coupling means for connecting together two shafts via an axial compensation element that accommodates an axial, angular, or lateral offset between said shafts.
In order to prevent damaging the compensation element as a result of high axial loads in traction while dismantling the coupling means, the coupling means include one or more abutments. Those abutments act axially in order to avoid damaging the compensation element under the effect of excessive axial traction.
In operation, pressure means apply an axial load to make the abutments inoperative.
Also known are documents U.S. Pat. No. 5,364,309 and GB 2 043 207.